Head-up-displays have long been used in military aircraft. First a symbol of the velocity vector of the aircraft is projected via a plate, which may be called a "reflecting glass". The symbol indicates the flight-path angle of the aircraft partly with the horizontal plane, flight-path angle .gamma., and partly with a meridian, track angle .chi.. The attitude angles, i.e. the Euler angles of the aircraft body relative to the horizontal plane, the roll angle .PHI. and pitch angle .theta., and also usually the heading angle .psi., are also generally presented by the corresponding symbols.
However, such head-up-displays only present straight lines for indicating pitch and roll attitudes and therefore represent only two of the three Euler angles required for a complete indication of the flight attitude. This means that the aircraft pilot during advanced flying manoeuvers such as dive and climb does not get an altogether complete picture of his three-dimensional flight attitude relative to the surroundings, which can lead to risks of insufficient understanding of the actual flight attitude when surrounding references are lacking for a short or long time, e.g. in clouds, fog or in the dark. Particularly in near vertical climbs or dives these indicators give a misleading picture of rotations in roll. There are known panel-mounted indicators of the kind described in the Swedish Pat. No.8300065-3, comprising e.g. an IN-platform controlled "three-axle" ball for all three Euler angles .PHI., .theta., and .psi. with latitude markings on the outside, distinguishable for up and down. These indicators require that the aircraft pilot lower his eyes down in the cabin to the front panel ("Head Down") and therefore lose the possibility of being constantly in contact with the surroundings.